Devices and techniques for integrated optical data communication

ABSTRACT

Devices and techniques for integrated optical data communication. A method of encoding symbols in an optical signal may include encoding a first symbol by injecting charge carriers, at a first rate, into a semiconductor device, such as a PIN diode. The method may also include encoding a second symbol by injecting charge carriers, at a second rate, into the semiconductor device. The first rate may exceed the second rate. A modulator driver circuit may include a resistive circuit coupled between supply terminal and drive terminals. The modulator driver circuit may also include a control circuit coupled between a data terminal and the resistive circuit. The control circuit may modulate a resistance of the resistive circuit by selectively coupling one or more of a plurality of portions of the resistive circuit to the drive terminal based on data to be optically encoded. In some embodiments, a modulator driver circuit and an optical modulator may be integrated on the same die or stacked (3D integrated) die and connected with through-oxide or through-silicon vias.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase of PCT/US2013/026375, filedFeb. 15 2013, and titled “DEVICES AND TECHNIQUES FOR INTEGRATED OPTICALDATA COMMUNICATION,” which claims the benefit under 35 U.S.C. § 119(e)of U.S. Provisional Patent Application Ser. No. 61/715,940, filed Oct.19, 2012, and titled, “INTEGRATED SIGNAL-CONDITIONING DRIVER CIRCUIT FORP-I-N SILICON-PHOTONIC MODULATORS,” each of which application are herebyincorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.ECCS0844994 awarded by the National Science Foundation and underContract No. W911NF-10-1-0412 awarded by the Army Research Office. Thegovernment has certain rights in this invention.

BACKGROUND

1. Field

The techniques and devices described herein relate generally tointegration of optical data communication technology and electroniccircuits, and relate particularly to an optical modulator driver circuitwhich is suitable for use with integrated silicon photonic interconnecttechnology and is compatible with a standard CMOS process.

2. Discussion of the Related Art

Many electronic devices use one or more integrated circuits to receive,store, process, and/or send data. An integrated circuit (“IC” or “chip”)may include a wafer of semiconductor material, such as silicon, on whichone or more electronic circuits have been fabricated by applying asequence of processing steps to the semiconductor wafer. Theseprocessing steps may include, for example, photolithographic patterning,material deposition, doping, annealing, material removal, and cleaning.For reasons that are understood by one of ordinary skill in the art(e.g., power dissipation, scalability, and/or cost of manufacturing),many ICs are fabricated using a standard CMOS (complementary metal-oxidesemiconductor) manufacturing process, rather than a customized CMOSprocess or a non-CMOS process.

An electronic device may use data communication technology to move datafrom one location to another within the device, or to exchange data withanother device. A variety of data communication technologies are known,including electrical and optical technologies. Electrical datacommunication technologies transport data by propagating electricalsignals through metal interconnects (e.g., wires). Optical datacommunication technologies transport data by propagating optical signals(e.g., light) through optical interconnects (e.g., waveguides).

In an optical data communication system, data may be encoded in anoptical signal by modulating one or more of the signal's properties,such as its phase, amplitude, frequency, or polarization. Suchmodulation may be achieved by changing an optical property of thewaveguide through which the optical signal propagates, such as thewaveguide's absorption coefficient or refractive index. The optical datacommunication system's performance (e.g., bandwidth-density andpower-density) depends on the rate at which data is encoded and theenergy dissipated during the encoding of each data symbol (e.g., eachbit).

Optical data communication technologies that propagate optical signalsthrough a silicon medium are known as silicon photonic systems. In asilicon photonic system, the plasma dispersion effect may be used tocontrol the concentration of free charge carriers in a semiconductordevice, thereby modulating the light carried by a nearby opticalwaveguide. The concentration of free-carriers may be controlled bycarrier injection, carrier depletion, or carrier accumulationtechniques. For example, injection-based modulation may be performedusing a PIN diode with a silicon waveguide embedded in its intrinsicregion. Forward-biasing the PIN diode causes carriers to be injectedinto the intrinsic region, thereby changing the silicon waveguide'srefractive index. As another example, depletion-based modulation may beperformed using a PN junction diode with a silicon waveguide embedded inthe PN junction region. Reverse-biasing the PN junction causes carriersto be removed from the PN junction region, thereby changing thewaveguide's refractive index. As yet another example, accumulation-basedmodulation may be performed using a device with an insulating layerbetween P and N regions of a diode (e.g., a MOS capacitor).

Although silicon's refractive index is only weakly dependent on theconcentration of free charge carriers, a ring resonator structure forenhancing this dependence has been proposed (Lipson, Nature 2004, p.1082). Use of such a ring resonator structure may facilitate low-poweroptical modulation in silicon.

BRIEF SUMMARY

The foregoing summary is provided by way of illustration and is notintended to be limiting.

According to an embodiment of the present disclosure, there is provideda method of encoding a first symbol and a second symbol in an opticalsignal carried by a waveguide. An optical property of the waveguidedepends on a concentration of charge carriers in a portion of asemiconductor device. The method includes encoding the first symbol inthe optical signal, wherein encoding the first symbol includes injectingcharge carriers, at a first rate, into the portion of the semiconductordevice; and encoding the second symbol in the optical signal, whereinencoding the second symbol includes injecting charge carriers, at asecond rate, into the portion of the semiconductor device. The firstrate is greater than the second rate.

According to another embodiment of the present disclosure, there isprovided an integrated circuit for controlling an optical modulator. Theoptical modulator is configured to encode data in an optical signalcarried by an optical waveguide. The optical modulator includes asemiconductor device. An optical property of the waveguide depends on aconcentration of charge carriers in a portion of a semiconductor device.The integrated circuit includes a modulator driver circuit configured tocontrol encoding of a first symbol in the optical signal by injectingcharge carriers into a portion of the semiconductor device at a firstrate, and to control encoding of a second symbol in the optical signalby injecting charge carriers into the portion of the semiconductordevice at a second rate. The first rate is greater than the second rate.

According to another embodiment of the present disclosure, there isprovided a method of fabricating an integrated circuit in amanufacturing process. The method includes, in a first layer of themanufacturing process, fabricating a body of a transistor of a modulatordriver circuit configured to control an optical modulator, thetransistor being configured to modulate a resistance between an opticalmodulator and a supply; in a second layer of the manufacturing process,fabricating a gate of the transistors of the modulator driver circuit;and fabricating an optical waveguide core in the first layer and/or thesecond layer of the manufacturing process.

According to another embodiment of the present disclosure, there isprovided an integrated circuit for controlling an optical modulator. Theoptical modulator is configured to encode data in an optical signalcarried by an optical waveguide. The optical modulator includes asemiconductor device. An optical property of the waveguide depends on aconcentration of charge carriers in a portion of a semiconductor device.The integrated circuit includes a modulator driver circuit including adata terminal for receiving data to be optically encoded; a supplyterminal for coupling to one or more supplies; a drive terminal forcoupling to the optical modulator; a resistive circuit (604) coupledbetween the supply terminal and the drive terminal, the resistivecircuit including a plurality of portions; and a control circuit coupledbetween the data terminal and the resistive circuit, the control circuitbeing configured to modulate a resistance of the resistive circuit byselectively coupling one or more of the plurality of portions of theresistive circuit to the drive terminal based on the data to beoptically encoded.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, identical or nearly identical components that areillustrated in various figures may be represented by a like referencecharacter. For purposes of clarity, not every component may be labeledin every drawing. The drawings are not necessarily drawn to scale, withemphasis instead being placed on illustrating various aspects of thetechniques and devices described herein.

FIG. 1 shows a block diagram of an optical data communication system,according to some embodiments;

FIG. 2A shows a schematic of a configuration of components including aring resonator 200, according to some embodiments;

FIG. 2B shows a schematic of a configuration of components including aring resonator 200 embedded in a diode, according to some embodiments;

FIG. 2C shows a cross section of a PIN diode along the line A-A,according to some embodiments;

FIG. 2D shows a cross section of a PN junction diode along the line A-A,according to some embodiments;

FIG. 3A shows a control signal suitable for controlling an opticalmodulator, according to some embodiments;

FIG. 3B shows another control signal suitable for controlling an opticalmodulator, according to some embodiments;

FIG. 3C shows yet another control signal suitable for controlling anoptical modulator, according to some embodiments;

FIG. 4 shows a block diagram of a modulator controller, according tosome embodiments;

FIG. 5 shows a block diagram of a modulator driver circuit, according tosome embodiments;

FIG. 6 shows a block diagram of a resistance modulation unit coupled toan optical modulator, according to some embodiments;

FIG. 7 shows a block diagram of a resistance modulation bias unit,according to some embodiments;

FIGS. 8A-8D show schematics of resistance modulation control units,according to some embodiments;

FIG. 9 shows a schematic of an inverter, according to some embodiments;

FIGS. 10A-10B show schematics of resistance modulation drive units,according to some embodiments;

FIGS. 11A-11B show block diagrams of modulator driver circuits,according to some embodiments;

FIG. 12 shows a flowchart of a method of encoding a first symbol and asecond symbol in an optical signal, according to some embodiments;

FIG. 13 illustrates components of an optical data communication systemin layers of a standard CMOS process, according to some embodiments; and

FIG. 14 shows a flowchart of a method of fabricating an integratedcircuit in a manufacturing process, according to some embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

The inventors has appreciated a need for an integrated optical datacommunication technology that is compatible with standard CMOSprocesses. Techniques are needed for increasing the communicationbandwidth and reducing the power dissipation of integrated circuits(ICs). Integrated optical data communication technologies—which mayprovide higher communication bandwidth and reduced power dissipationthan the electrical data communication technologies used by mostconventional ICs—have been proposed. However, the commercial viabilityof an IC structure may depend greatly on whether the structure can bemanufactured in a standard CMOS process, and previously proposedtechniques for integrating optical data communication technologies intointegrated circuits (ICs) require customized processing steps and/ormaterials that are not used in standard CMOS manufacturing processes.For example, incorporation of optical data communication technologiesinto ICs through hybrid or heterogeneous process technologies has beenproposed. As another example, a customized CMOS process for fabricatingICs with optical data communication technology has been proposed, butthe proposed process requires a buried oxide (BOX) layer that is atleast an order of magnitude thicker than the BOX layers used in standardCMOS processes. These techniques tend to increase the cost offabricating the IC, suffer from large packaging parasitics, and/ordegrade transistor performance.

Described herein are devices and associated techniques that can be usedto provide high-bandwidth, low-power, optical data communication systemson integrated circuits, including integrated circuits fabricated withstandard CMOS processes. Operating the disclosed devices in accordancewith the disclosed methods may enable an integrated optical datacommunication system, such as an integrated silicon photonic system, tooperate at a high bandwidth (e.g., hundreds of Gb/s, Tb/s, etc.) andwith low energy dissipation (e.g., 1.2 pJ/b).

According to an aspect of the present disclosure, a modulator drivercircuit may be used to electrically control an optical modulator, suchas a silicon photonic modulator that is controlled by the free-carrierconcentration in a semiconductor device. In some embodiments, themodulator driver circuit uses resistance modulation to electricallycontrol the drive strength of the optical modulator (e.g., the currentthrough a component of the modulator). In some embodiments, themodulator driver circuit is suitable for fabrication in standard CMOSprocesses, without requiring process customizations or foundry changes.For example, embodiments of the driver circuits are suitable forfabrication in sub-130 nm standard CMOS processes, such as 22 nm or 45nm bulk and silicon-on-insulator (SOI) processes.

According to another aspect of the present disclosure, pre-emphasis ofthe drive strength of an optical modulator may be used to speed up themodulator's switching rate (e.g., the rate at which the modulatorswitches from optically encoding one symbol to optically encodinganother symbol). In some embodiments, an optical modulator's drivestrength may control the rate at which free charge carriers are injectedinto, depleted from, or accumulated in a portion of the opticalmodulator, and the concentration of free charge carriers may determinewhether the light is modulated to encode a 0-bit or a 1-bit. In someembodiments, a low-to-high transition of the modulator's drive strengthmay be pre-emphasized, thereby accelerating injection of free chargecarriers and speeding up the modulator's transition from a first state(e.g., a state in which light is modulated to encode a 0-bit) to asecond state (e.g., a state in which light is modulated to encode a1-bit). In some embodiments, the a high-to-low transition of themodulator's drive strength may be pre-emphasized, thereby acceleratingremoval of free charge carriers and speeding up the modulator'stransition from the second state (e.g., the 1-bit encoding state) to thefirst state (e.g., the 0-bit encoding state). In some embodiments,pre-emphasis of the modulator's drive strength may be controlled bymodulating a resistance of a modulator driver circuit.

According to another aspect of the present disclosure, the opticalmodulator's mode of operation may be controlled to speed up themodulator's switching rate. For example, in some embodiments, acomponent of the optical modulator (e.g., a PIN diode) may beforward-biased during the 1-bit encoding state and weakly forward-biasedduring the 0-bit encoding state. Forward-biasing may refer to biasingthe component at a voltage above its threshold voltage, and weaklyforward-biasing may refer to biasing the component in the forwarddirection at a voltage below its threshold voltage. This technique mayaccelerate the modulator's transitions from a first encoding state to asecond encoding state (e.g., from the 0-bit encoding state back to the1-bit encoding state), by reducing the number of additional free chargecarriers that must be injected into the optical modulator to bring aboutthe transition to the second encoding state.

According to another aspect of the present disclosure, a siliconphotonic data communication system may be fabricated in a standard CMOSprocess. In some embodiments, bodies of transistors of a modulatordriver circuit and portions of a silicon waveguide may be fabricated ina body-silicon layer at the front end of a CMOS process. In someembodiments, gates of transistors of a modulator driver circuit andportions of a silicon waveguide may be fabricated in a gate-polysiliconlayer at the front end of a CMOS process. The transistors of themodulator drive circuit may modulate a resistance between a voltagesupply and an optical modulator component, such as a PIN diode.

The aspects described above, as well as additional aspects, aredescribed further below. These aspects may be used individually, alltogether, or in any combination of two or more, as the technology is notlimited in this respect.

Optical Data Communication System

FIG. 1 shows a block diagram of an optical data communication system100, according to some embodiments. The system includes a light source102, an optical waveguide 104, an optical modulator 106, a modulatorcontroller 108, and a receiver 110. In some embodiments, one or morecomponents of optical data communication system 100 may be integratedon-chip with one another. The integration may be monolithic orheterogeneous. As just one example, optical waveguide 104, modulator106, modulator controller 108, and receiver 110 may be monolithicallyintegrated on-chip through a standard CMOS process, and an off-chiplight source 102 may be coupled to the other components of optical datacommunication system 100 through couplers (e.g., vertical grating oredge couplers). However, embodiments are not limited in this respect.Components of data communication system 100 may be implemented andcoupled together by any means known of one to ordinary skill in the artor otherwise suitable for implementing and coupling components of anoptical data communication system.

Light source 102 provides light (e.g., an optical signal) whichpropagates through the optical waveguide 104. The light source 102 maybe, for example, a laser, a light emitting diode (LED), or any otherlight source known to one of ordinary skill in the art or otherwisesuitable for use in an optical data communication system. In someembodiments, light source 102 may provide light having wavelengthsbetween approximately 1260 nm to 1350 nm, or wavelengths betweenapproximately 1560 nm to 1630 nm. However, embodiments of light source102 are not limited in this regard. Light source 102 may provide lighthaving any wavelengths suitable for propagation through an opticalwaveguide 104 (e.g., a silicon waveguide), including (but not limitedto) wavelengths between 1100 nm and 1600 nm.

Optical waveguide 104 is a structure which guides the light provided bythe light source 102. Optical waveguide 104 may be, for example, a stripwaveguide, a rib waveguide, a segmented waveguide, and/or a photoniccrystal waveguide. The transmission loss of optical waveguide 104 maybe, for example, less than 3 dB/cm, less than 2.5 dB/cm, less than 2.0dB/cm, less than 1.5 dB/cm, or less than 1.0 dB/cm. In some embodiments,optical waveguide 104 may be formed from silicon, polysilicon, and/orother silicon-based materials. Embodiments of optical waveguide 104 thatare formed from silicon may be formed in the body silicon layer of thefront-end of a standard CMOS process. Embodiments of optical waveguide104 that are formed from polysilicon may be formed in thegate-polysilicon layer of the front-end of a standard CMOS process. Inembodiments where the optical waveguide is formed from a silicon-basedmaterial, the optical data communication system 100 may be a siliconphotonic system. In some embodiments, the transmission loss of anoptical waveguide (e.g., silicon) may be reduced to an acceptable level(e.g., less than 3 dB/cm) using techniques known to one of ordinaryskill in the art, such as partially undercutting the waveguide (i.e.,removing some of the silicon surrounding the waveguide so that theportions of the waveguide are suspended) and filling the area around thewaveguide with a substance that has low transmission loss, or removingthe substrate and covering the waveguide with a material that has lowtransmission loss.

Embodiments of optical waveguide 104 may propagate a single opticalsignal at a time, or propagate multiple optical signals of differentwavelengths simultaneously. Simultaneous propagation of multiple opticalsignals of different wavelengths may be carried out in accordance with awavelength-division multiplexing (WDM) protocol. When multiple opticalsignals of different wavelengths simultaneously propagate throughoptical waveguide 104, the communication bandwidth of the opticalwaveguide may exceed 10 Gb/s, 100 Gb/s, or 1 Tb/s.

Optical modulator 106 encodes data in the light propagating throughwaveguide 104 by modulating one or more properties of the light, such asthe light's phase, amplitude, frequency, or polarization. Someembodiments may modulate the light by changing an optical property ofwaveguide 104, such as the waveguide's absorption coefficient orrefractive index. Embodiments of optical modulator 106 may controlchanges in the optical properties of waveguide 104 using electro-opticmodulation, acousto-optic modulation, magneto-optic modulation,thermo-optic modulation, mechano-optic modulation, or any othermodulation technique known to one of ordinary skill in the art orotherwise suitable for controlling a waveguide's optical properties.

In some embodiments, optical modulator 106 may modulate the lightcarried by waveguide 104 to encode symbols from an N-symbol alphabet,where N is a finite number. Each symbol in the modulator's N-symbolalphabet may correspond to a distinct encoding state of the modulator,such that when the modulator is in a given encoding state, the light ismodulated to encode the symbol that corresponds to that encoding state.For example, embodiments of optical modulator 106 may modulate the lightto encode a 2-symbol (binary) alphabet by modulating the light to encodea first symbol (e.g., a 1-bit) when the modulator is in a first encodingstate, and modulating the light to encode a second symbol (e.g., a0-bit) when the modulator is in a second encoding state.

Embodiments of optical modulator 106 may include any structure thatmodulates light passing through optical waveguide 104 in response to theconcentration of free carriers in a portion of the optical modulator.For example, embodiments of optical modulator 106 may include acarrier-concentration controller, such as a MOS capacitor, a PIN diode,or a PN junction diode. In some embodiments, the drive strength of theoptical modulator (e.g., the level of the current entering or leaving aterminal of the optical modulator, or the level of the current passingthrough the modulator's carrier-concentration controller) may determinethe rate at which free carriers are injected into, depleted from, oraccumulated in a portion of the optical modulator, thereby controllingthe modulator's operation.

Optical modulator 106 may operate in one or more operating modes. Insome embodiments, the modulator's operating modes may correspond tooperating modes of a carrier-concentration controller included in theoptical modulator. For example, some embodiments of optical modulator106 may have two operating modes, corresponding to forward-biased orreverse-biased operation of a carrier-concentration controller, such asa PIN diode or a PN junction diode in which a ring resonator isembedded. Some embodiments of optical modulator 106 may have threeoperating modes, corresponding to forward-biased, weakly forward-biased,or reverse-biased operation of a carrier-concentration controller. Someembodiments of optical modulator 106 may have four operating modes,corresponding to strongly forward-biased, forward-biased, weaklyforward-biased, or reverse-biased operation of a carrier-concentrationcontroller. In some embodiments, steady-state operation of the opticalmodulator in one or more operating modes may correspond to one or moreof the modulator's encoding states.

Modulator controller 108 controls the operation of optical modulator106. In some embodiments, modulator controller 108 may receive datainput indicative of data to be encoded in the light propagating throughwaveguide 104. Based on the data input, modulator controller 108 mayprovide a control input to modulator 106, thereby causing modulator 106to modulate the light propagating through waveguide 104 such that thelight encodes the data. Modulator controller 108 may provide the controlinput to modulator 106 in any suitable form, including but not limitedto an electrical signal, an acoustic signal, a magnetic signal, or athermal signal.

In some embodiments, optical modulator 106 may include a ring resonator200. Some embodiments of ring resonator 200 may be implemented in thebody-silicon layer and/or gate (or resistor, or sacrificial) polysiliconlayer in the front-end of a standard CMOS process (or thin-BOX SOI CMOSprocess), or as amorphous, crystalline or poly silicon layers depositedin the back-end of a standard CMOS process (or thin BOX SOI CMOSprocess). FIG. 2A schematically illustrates a configuration ofcomponents including a ring resonator 200, according to someembodiments. In the embodiment of FIG. 2A, modulator controller 108controls the modulation of light propagating through waveguide 104 bytuning the resonance frequency of ring resonator 200. In the embodimentof FIG. 2A, waveguide 104 is a poor transmitter (e.g., exhibits a lowtransmission coefficient) for light having a wavelength that matches theresonance wavelength of ring resonator 200, but a good transmitter(e.g., exhibits a high transmission coefficient) for light having otherwavelengths. In other words, in the embodiment of FIG. 2A, one or moreproperties of the light propagating through waveguide 104 depend onwhether the light's wavelength matches the resonance wavelength of ringresonator 200. Thus, tuning the resonance wavelength of ring resonator200 to the wavelength of the light in waveguide 104 and detuning theresonance wavelength of ring resonator 200 from the wavelength of thelight in waveguide 104 may modulate the light's properties, therebyencoding data in the light.

In some embodiments, electro-optic effects may be used to tune theresonance wavelength of ring resonator 200. FIG. 2B schematicallyillustrates a configuration of components which may facilitateelectrically-controlled modulation of the light in waveguide 104,according to some embodiments. In the embodiment of FIG. 2B, ringresonator 200 is implemented using a strip-loaded (rib) waveguide 204that is embedded in a diode (202, 204, 206), such that the resonancewavelength of ring resonator 200 depends on the concentration of freecharge carriers in waveguide 204. Embodiments of waveguide 204 mayfunction as carrier-injection phase shifters.

In the embodiment of FIG. 2B, modulator controller 108 is implementedusing a modulator driver circuit 208. Some embodiments of modulatordriver circuit 208 may control the drive strength applied to theterminals of the diode (202, 204, 206) in which waveguide 204 isembedded. In this manner, embodiments of modulator driver circuit 208may electrically control the concentration of free charge carriers in aportion of the diode, thereby controlling the resonance wavelength ofring resonator 200, thereby controlling modulation of the light inwaveguide 104.

In some embodiments, the diode in which waveguide 204 of ring resonator200 is embedded may be PIN diode (e.g., an enhancement-mode PIN diode).FIG. 2C, which shows a cross section of a diode along line A-A of FIG.2B, illustrates an embodiment in which ring resonator 200 is embedded ina PIN diode 201. As shown in FIG. 2C, the PIN diode includes aheavily-doped n-type diffusion region 202, a heavily-doped p-typediffusion region 206, and an intrinsic semiconductor region 210 betweenthe p-type and n-type regions. The PIN diode of FIG. 2B may switchbetween a forward-biased state and a reverse-biased state at a rate ofapproximately 200 MHz. In some embodiments, the diffusion regions 202and 206 and intrinsic semiconductor region 210 may be implemented in thebody-silicon layer of the front-end of a standard CMOS process. In someembodiments, intrinsic semiconductor region 210 may be overlain by apolysilicon layer 212, which may be implemented in the gate-polysiliconlayer of the front-end of a standard CMOS process. Together, portions ofthe intrinsic semiconductor region 210 and the optional polysiliconlayer 212 may form the waveguide 204 of ring resonator 200. FIG. 2C alsoillustrates an embodiment of modulator driver circuit 208, whichcontrols the concentration of charge carriers in the ring resonator'swaveguide by controlling the drive strength of the PIN diode throughterminals coupled to the diode's p-type and n-type regions.

In some embodiments, the diode in which waveguide 204 of ring resonator200 is embedded may be a PN junction diode (e.g., a depletion-mode PNdiode). FIG. 2D, which shows a cross section of another diode along lineA-A of FIG. 2B, illustrates an embodiment in which ring resonator 200 isembedded in a PN junction diode 211. As shown in FIG. 2D, the PNjunction diode includes a heavily-doped n-type diffusion region 202, ann-type diffusion region 222, a p-type diffusion region 226, and aheavily-doped p-type diffusion region 206. In some embodiments, thediffusion regions 202, 206, 222, and 226 may be implemented in thebody-silicon layer of the front-end of a standard CMOS process. In someembodiments, diffusion regions 222 and 226 may be overlain by apolysilicon layer 212, which may be implemented in the gate-polysiliconlayer of the front-end of a standard CMOS process (or in the depositedamorphous, crystalline and/or poly silicon layers in the backend of aCMOS process). Together, portions of the diffusion regions 222 and 226and optional polysilicon layer 212 may form the waveguide 204 of ringresonator 200. FIG. 2D also illustrates an embodiment of modulatordriver circuit 208, which controls the concentration of charge carriersin the ring resonator's waveguide by controlling the drive strength ofthe PN junction diode through terminals coupled to the diode'sheavily-doped diffusion regions.

Returning to FIG. 1, optical data communication system 100 also includesan optical receiver 110. Optical receiver 110 may receive light encodedwith data through waveguide 104, and convert the optically-encoded datainto electrical signals. Embodiments of optical receiver 110 may includeany structure that detects light, including (but not limited to) aphotodiode or a photogate.

Controlling an Optical Modulator

Embodiments of modulator controller 108 may control the operating modeand/or encoding state of optical modulator 106 by controlling theconcentration of free carriers in a region of a semiconductor device(e.g., by controlling the rate at which free carriers are injected,discharged, and/or accumulated). In some embodiments, the concentrationof free carriers may be controlled by a signal provided in a portion ofoptical modulator 106. FIG. 3A shows an example 300 a of a signal thatcontrols the concentration of free carriers in an optical modulator 106during optical encoding of data. In the example of FIG. 3A, signal 300 aillustrates the drive strength of the optical modulator during a periodof operation. However, embodiments are not limited in this regard. Theconcentration of free carriers may be controlled by any type ofelectrical signal, such as a current, voltage, or power signal.

In the example of FIG. 3A, modulator controller 108 switches the levelof signal 300 a between a first signal level 392 and a second signallevel 394. When the signal's level is above a threshold level 332,optical modulator 106 operates in a first mode of operation 302. Whenthe control signal's level is below the threshold level 332, opticalmodulator 106 operates in a second mode of operation 304. While theoptical modulator 106 is in the first mode of operation 302, themodulator enters a first encoding state 312, in which the modulatorencodes a first symbol in the light carried by waveguide 104. Two ormore consecutive instances of the first symbol may be optically encodedwhile the modulator remains in the first encoding state 312. While theoptical modulator 106 is in the second mode of operation 304, themodulator enters a second encoding state 314, in which the modulatorencodes a second symbol in the light carried by waveguide 104. Two ormore consecutive instances of the second symbol may be optically encodedwhile the modulator remains in the second encodings state 314.

As described above, signal 300 a of FIG. 3A illustrates the drivestrength of some embodiments of optical modulator 106, during a periodof operation. The optical modulator may include a PIN diode 201 in whicha ring resonator 200 is embedded. When the drive strength exceeds athreshold level 332, the PIN diode 201 may operate in a forward-biasedmode 302, causing free charge carriers to be injected into the diode'sintrinsic region 210 (e.g., at a rate that exceeds the rate of freecarrier recombination). When the concentration of free charge carriersin the diode's intrinsic region exceeds a threshold concentration,optical modulator 106 may enter a first encoding state 312, in whichring resonator 200 causes a 1-bit to be encoded in the light carried bywaveguide 104. When the drive strength falls below threshold level 332,PIN diode 201 may operate in a reverse-biased mode 304, in which theconcentration of charge carriers in the diode's intrinsic region 210decreases because free carriers are discharged from the intrinsic regionor additional free-carriers are not injected into the intrinsic regionto compensate for free-carrier recombination. When the concentration offree charge carriers in the diode's intrinsic region falls below athreshold concentration, optical modulator 106 may enter a secondencoding state 314, in which ring resonator 200 causes a 0-bit to beencoded in the light carried by waveguide 104.

Embodiments of modulator driver circuit 208 may control the drivestrength of the optical modulator 106 according to the techniquesillustrated in FIG. 3A. For example, embodiments of modulator drivercircuit 208 may adjust the drive strength of the optical modulatorbetween levels 392 and 394, respectively. Drive strength level 392 maybe, for example, 100-200 Ohm from a 1.0-1.5V supply (or 5-10 mA). Drivestrength level 394 may be, for example, 100-1 kOhm from a reverse biasedsupply of 1.0-1.5V (or 1-10 mA). In some embodiments, modulator drivercircuit 208 may be cutoff (e.g., enter a high-impedance state) after theoptical modulator enters second encoding state 314. Cutting offmodulator driver circuit 208 in this manner may reduce the powerdissipation of the optical data communication system. Embodiments ofmodulator driver circuit 208 which control the drive strength of opticalmodulator 106 according to the techniques illustrated in FIG. 3A may becapable of controlling a modulator with a ring resonator 200 embedded ina PIN diode 201 to encode binary data in an optical signal at a rate ofapproximately 600 Mb/s, with energy dissipation of approximately 4.2pJ/bit.

Some embodiments of modulator controller 108 may perform pre-emphasis(e.g., sub-symbol pre-emphasis) on the drive strength of opticalmodulator 106 during signal transitions. In some embodiments,pre-emphasizing the modulator's drive strength may increase the rate atwhich optical modulator 106 encodes data in an optical signal. In someembodiments, pre-emphasis may be performed by modulating the resistanceof a modulator controller connected to a fixed supply voltage, ratherthan by adjusting the supply voltage or switching between multiplesupplies.

Signal 300 b of FIG. 3B illustrates the drive strength of someembodiments of optical modulator 106, during a period of operation. Theoptical modulator may include a PIN diode 201 in which a ring resonator200 is embedded. In the example of FIG. 3B, the modulator's drivestrength is pre-emphasized during signal transitions. For example,during a transition from level 344 to level 342, the drive strength ispre-emphasized to level 350 before stabilizing at level 342. When thedrive strength is at level 350, charge carriers may be injected into thediode's intrinsic region at a rate that exceeds the rate of carrierrecombination, thereby rapidly increasing the concentration of chargecarriers in the intrinsic region. When the drive strength is at level342, charge carriers may be injected at a rate sufficient to maintain orslowly increase the elevated concentration of charge carriers.

Likewise, during a transition from level 342 to level 344, the drivestrength is pre-emphasized to level 345 before stabilizing at level 344.When the drive strength is at level 345, charge carriers may be rapidlydischarged from the diode's intrinsic region, thereby decreasing theconcentration of charge carriers. When the drive strength is at level344, free carriers may be discharged from the intrinsic region oradditional free-carriers may not be injected into the intrinsic regionto compensate for carrier recombination. Pre-emphasizing the modulator'sdrive strength during transitions may increase the rate of change of thefree carrier concentration in the PIN diode's intrinsic region, therebyincreasing the rate at which optical modulator 106 switches betweenencoding states.

In the example of FIG. 3B, modulator controller 108 adjusts themodulator drive strength relative to a first threshold level 332 and asecond threshold level 333. When the drive strength is above the firstthreshold level 332, optical modulator 106 operates in a first mode ofoperation 306 (e.g., a forward-biased mode). When the drive strength isabove the second threshold level 333, optical modulator 106 may operatein a second mode of operation 308 (e.g., a ‘strongly’ forward-biasedmode 303). In some embodiments, the strongly forward-biased mode 303 maybe characterized by the injection of minority charge carriers intooptical modulator 106 (e.g., into the intrinsic region of a PIN diode)at a rate that exceeds the rate of carrier recombination. When the drivestrength is below the first threshold level 332, optical modulator 106operates in a third mode of operation 304 (e.g., a reverse biased mode).

Embodiments of modulator driver circuit 208 may control the drivestrength of optical modulator 106 according to the techniquesillustrated in FIG. 3B. For example, embodiments of modulator drivercircuit 208 may control the modulator's drive strength such thatpre-emphasis (e.g., sub-bit pre-emphasis) of the drive strength occursduring transitions from level 342 to level 344 (e.g., during thetransition from the 1-bit encoding state to the 0-bit encoding state),and/or during transitions from level 344 to level 342 (e.g., during thetransition from the 0-bit encoding state to the 1-bit encoding state).Pre-emphasis may accelerate the injection of charge carriers during atransition from a first encoding state to a second encoding state, andmay accelerate removal of charge carriers during a transition from thesecond encoding state to the first encoding state. The pre-emphasizeddrive strength level 350 may be, for example, 100 Ohm to 400 Ohm from1.0-1.5V supplies (or 10 mA to 2 mA). The pre-emphasized drive strengthlevel 345 may be, for example, 100 Ohm to 1 kOhm from 1.0-1.5V reversebias supply. Embodiments of modulator driver circuit 208 which controlthe drive strength of optical modulator 106 according to the techniquesillustrated in FIG. 3B (e.g., by pre-emphasizing the drive strength) maybe capable of controlling a modulator with a ring resonator 200 embeddedin a PIN diode 201 to encode binary data in an optical signal at a rateof approximately 1 Gb/s, with energy dissipation of approximately 3pJ/b.

Some embodiments of modulator controller 108 may prime optical modulator106 for a fast transition from one encoding state to another. Primingthe optical modulator may include causing the modulator to switch from afirst operating mode associated with a first encoding state to a secondoperating mode associated with the first encoding state, where themodulator can transition from the second operating mode to a thirdoperating mode associated with a second encoding state more quickly thanthe modulator can transition from the first operating mode to the thirdoperating mode. In some embodiments, priming the modulator for a fasttransition to an encoding state may increase the rate at which theoptical modulator encodes data in an optical signal.

Signal 300 c of FIG. 3C illustrates the drive strength of someembodiments of optical modulator 106, during a period of operation. Theexample of FIG. 3C illustrates pre-emphasis and priming techniques.Drive strength levels 350 and 345 of signal 300 c illustratepre-emphasis. The benefits of drive strength pre-emphasis are discussedabove with respect to FIG. 3B. Drive strength level 352 of signal 300 cillustrates priming of the optical modulator to facilitate fasterswitching between encoding states.

In the example of FIG. 3C, modulator controller 108 switches themodulator drive strength among levels 342, 345, and 352. When the drivestrength is at level 342, charge carriers may be injected at a ratesufficient to maintain or increase a high concentration of chargecarriers, as described above with reference to FIG. 3B. When the drivestrength is at level 345, charge carriers may be rapidly discharged fromthe diode's intrinsic region, thereby decreasing the concentration ofcharge carriers. When the drive strength is at level 352, chargecarriers may be slowly injected into the diode's intrinsic region (e.g.,at a rate that is lower than or approximately equal to the rate ofcarrier recombination), thereby maintaining a low concentration ofcharge carriers. In addition, the resistance of the diode may be lowerwhen the drive strength is at level 352 than when the drive strength isat level 345.

In the example of FIG. 3C, modulator controller 108 adjusts themodulator drive strength relative to a first threshold level 332, asecond threshold level 334, and a third threshold level 333. When thedrive strength is above the first threshold level 334, optical modulator106 operates in a first mode of operation 302 (e.g., a forward-biasedmode). When the drive strength is below the second threshold level 334but above the second threshold level 332, optical modulator 106 operatesin a second mode of operation 308 (e.g., a ‘weakly’ forward-biased modeor ‘sub-threshold’ forward-biased mode). When the drive strength isabove the third threshold level 333, optical modulator 106 may operatein a third mode of operation 303 (e.g., a ‘strongly’ forward-biased mode303). When the drive strength is below the second threshold level 332,optical modulator 106 operates in a fourth mode of operation 304 (e.g.,a reverse biased mode).

In the example of FIG. 3C, while optical modulator 106 is in the firstmode of operation 302, the modulator enters a first encoding state 312,in which the modulator encodes a first symbol (e.g., a 1-bit) in thelight carried by waveguide 104. Two or more consecutive 1-bits may beoptically encoded while the modulator remains in the first encodingstate 312. While optical modulator 106 is in the fourth mode ofoperation 304, the modulator enters a second encoding state 314, inwhich the modulator encodes a second symbol (e.g., a 0-bit) in the lightcarried by waveguide 104. Two or more consecutive 0-bits may beoptically encoded while the modulator remains in the second encodingsstate 314 and the fourth operating mode 304. However, when switchingfrom fourth mode of operation 304 to first mode of operation 302, themodulator's switching speed may not be sufficiently fast for someapplications. Thus, while optical modulator remains in the secondencoding state 314, modulator controller 108 may adjust the modulator'sdrive strength to a level between the first threshold 334 and the secondthreshold 332, causing the optical modulator to transition to secondoperating mode 308. Two or more consecutive 0-bits may be opticallyencoded while the modulator remains in the second encoding state 314 andthe second operating mode 308. Optical modulator 106 may be configuredto switch from second encoding state 314 to first encoding state 312more quickly when the modulator is in second operating mode 308 thanwhen the modulator is in fourth operating mode 304 (e.g., because theresistance between the modulator's terminals is lower in the secondoperating mode than in the fourth operating mode).

In some embodiments, priming may be applied to an optical modulatorwhich includes a PIN diode 201 in which a ring resonator 200 isembedded. When the drive strength exceeds a first threshold level 334(e.g., 100-500 Ohm at 1.0-1.5V supply, or 1-10 mA), the PIN diode 201may operate in a forward-biased mode 302 and/or strongly forward-biasedmode 303, causing free charge carriers to be injected into the diode'sintrinsic region 210 (e.g., at a rate that exceeds or approximatelyequals the rate of free-carrier recombination). When the concentrationof free charge carriers in the diode's intrinsic region exceeds athreshold concentration (e.g., 1 e19 cm⁻³ to 10 e19 cm⁻³), opticalmodulator 106 may enter a first encoding state 312, in which ringresonator 200 causes a 1-bit to be encoded in the light carried bywaveguide 104. When the drive strength is below the first thresholdlevel 334 but above a second threshold level 332, the PIN diode 201 mayoperate in a weakly forward-biased mode 308, such that free chargecarriers are either not injected into the diode's intrinsic region 210,or are injected at a rate less than or approximately equal to the rateat which charge carriers in the intrinsic region recombine. Thus, theconcentration of free charge carriers may remain roughly constant ordecrease when the PIN diode is weakly forward-biased. For example, theconcentration of charge carriers may be between 0.5 e18 cm⁻³ and 5 e18cm⁻³ when the PIN diode is weakly forward-biased. When the drivestrength is below the second threshold level 332, the PIN diode 201 mayoperate in a reverse-biased mode 304, in which the concentration ofcharge carriers in the diode's intrinsic region 210 decreases, becausecharge carriers are discharged from the intrinsic region or additionalfree-carriers are not injected to replace free carriers that recombine.When the concentration of free charge carriers in the diode's intrinsicregion falls below a threshold concentration (e.g., 0 to 0.5 e17 cm⁻³),optical modulator 106 may enter a second encoding state 314, in whichring resonator 200 causes a 0-bit to be encoded in the light carried bywaveguide 104.

The time required by embodiments of optical modulator 106 to transitionfrom the a first encoding state (e.g., a 0-bit encoding state) to asecond encoding state (e.g., a 1-bit encoding state) may depend in parton the concentration of free charge carriers in the intrinsic region ofPIN diode 201 during the first encoding state. If the concentration ofcharge carriers in the first encoding state is low, the transition tothe second encoding state may have a relatively long duration. Bycontrast, if the concentration of charge carriers in the first encodingstate is higher, the transition to the second encoding state may bequicker.

Embodiments of modulator driver circuit 208 may control the drivestrength of optical modulator 106 according to the techniquesillustrated in FIG. 3C. For example, embodiments of modulator drivercircuit 208 may control the modulator's drive strength such thatpre-emphasis (e.g., sub-bit pre-emphasis) of the drive strength occursduring transitions between encoding states, and/or prime opticalmodulator 106 for fast transitions between encoding states. In someembodiments, modulator driver circuit 208 may prime optical modulator106 for a fast transition between states by switching the modulator to amode in which free carriers are injected at a rate less than orapproximately equal to the rate of free-carrier recombination, such as aweakly-forward biased mode, after the modulator enters an encodingstate.

Embodiments of modulator driver circuit 208 which control the drivestrength of optical modulator 106 according to the techniquesillustrated in FIG. 3C (e.g., by pre-emphasizing the drive strengthduring transitions between encoding states, and/or by priming themodulator for fast transitions between encoding states) may be capableof controlling a modulator with a ring resonator 200 embedded in a PINdiode 201 to encode binary data in an optical signal at a rate ofapproximately 2.5-5.0 Gb/s, with energy dissipation of less than 5pJ/bit, less than 2 pJ/bit, less than 1.5 pJ/bit, less than 1.25 pJ/bit,or less than 1 pJ/bit. In addition, a modulator 106 controlled in thismanner may achieve extinction ratios of better than 3 dB at insertionloss of 3 dB, using supply voltages available in standard CMOSprocesses. The extinction ratio may improve as the insertion loss isincreased.

The signal levels and shapes illustrated in FIGS. 3A-3C are non-limitingexamples. In some embodiments, modulator controller 108 may beprogrammable, such that the levels and shape of the modulator's drivestrength characteristic are adjustable. For example, embodiments ofmodulator controller 108 may allow programmable control of pre-emphasis,such that the optical modulator's drive strength is pre-emphasizedduring some, all, or no transitions between operating modes or encodingstates. As another example, embodiments of modulator controller 108 mayallow programmable control of priming, such that the priming of theoptical modulator's operating mode is performed before some, all, or notransitions between encoding states. Embodiments of modulator controller108 may also allow general programmable control of the opticalmodulator's drive strength. Such general programmable control may permita user to tune the modulator controller, e.g., to account for processvariation or to make tradeoffs between communication bandwidth, opticalloss and power dissipation.

Embodiments of a Modulator Controller

FIG. 4 shows a block diagram of a modulator controller 108, according tosome embodiments. The modulator controller 108 of FIG. 4 includes aresistance modulation unit (RMU) 400, which has a data terminal 402, asupply terminal 406, a ground terminal 407, a control terminal 404, anda configuration terminal 408.

Data terminal 402 may be for receiving data to be optically encoded byan optical modulator. Ground terminal 407 may be for coupling resistancemodulation unit (RMU) 400 to ground. Control terminal 404 may be forcoupling to an optical modulator 106. Configuration terminal 408 may befor receiving configuration signals associated with programmable controlof the RMU's operation. For example, configuration terminal 408 may beconfigured to permit programmable control of the drive strength of anoptical modulator to which modulator controller 108 is coupled.

Supply terminal 406 may be for coupling resistance modulation unit (RMU)400 to one or more supplies, such as a power supply, voltage source, orcurrent source. For example, in some embodiments supply terminal 406 maybe coupled to two voltage supplies VDD and HVDD. In some embodiments,VDD and HVDD may be a core logic power supply and an input/output (I/O)power supply of an integrated circuit fabricated in a standard CMOSprocess. VDD may have a nominal voltage, for example, between 1.1V and1.3V. HVDD may have a nominal voltage, for example, of 1.8V. However,embodiments are not limited in this regard. Supply terminal 406 may befor coupling to any on-chip or off-chip supply.

In some embodiments, RMU 400 modulates a resistance between supplyterminal 406 and control terminal 404, and/or modulates a resistancebetween control terminal 404 and ground terminal 407, to control opticalencoding of data received on data terminal 402. Embodiments ofresistance modulation techniques that use one or more fixed suppliesassociated with a standard CMOS process may be compatible with astandard CMOS process. By contrast, resistance modulation techniquesthat use supply switching or supply modulation may not be compatiblewith a standard CMOS process.

FIG. 5 shows a block diagram of a modulator driver circuit 208,according to some embodiments. The modulator driver circuit 208 of FIG.5 includes a resistance modulation unit (RMU) 400, which has a dataterminal 402, a supply terminal 406, a ground terminal 407, aconfiguration terminal 408, and control terminals 404 a and 404 b. Thedata, supply, ground, and configuration terminals and their associatedfunctions are described above with reference to FIG. 4. Controlterminals 404 a and 404 b may be for controlling an optical modulator106. Embodiments of RMU 400 may use resistance modulation to controloptical encoding of data received on data terminal 402. Embodiments ofRMU 400 may modulate a resistance between control terminal 404 a andsupply terminal 406, a resistance between control terminal 404 a andground terminal 407, a resistance between control terminal 404 b andsupply terminal 406, and/or a resistance between control terminal 404 band ground terminal 407.

Embodiments of modulator driver circuit 208 may selectively providevoltages of positive or negative polarity across control terminals 404 aand 404 b. For example, resistance modulation unit (RMU) 400 may, at onetime, provide a higher potential at terminal 404 a than at terminal 404b. At another time, RMU 400 may provide a higher potential at terminal404 b than at terminal 404 a. In this manner, RMU 400 may selectivelyapply a positive or negative voltage to an optical modulator 106 coupledto control terminals 404 a and 404 b.

FIG. 6 shows a block diagram of resistance modulation unit (RMU) 400coupled to an optical modulator 106, according to some embodiments. RMU400 may include one or more resistance modulation bias units (RM biasunits) 500. The one or more RM bias units may be used to control thedrive strength of an optical modulator coupled to the control terminals.For example, different RM bias units may be used to provide voltages ofdifferent polarities to an optical modulator. As another example,different RM bias units may be used to adjust an optical modulator'sdrive strength. In some embodiments, signals provided by two or more RMbias units 500 on their respective control lines 410 and 411 may becombined to control the drive strength of an optical modulator.

Each of the RMU's one or more RM bias units 500 may be coupled to dataterminal 402, supply terminal 406, ground terminal 407, configurationterminal 408, and control lines 410 and 411. The data, supply, ground,and configuration terminals and their associated functions are describedabove with reference to FIG. 4. Control lines 410 and 411 may be forcoupling two or more RM bias units 500 to each other (e.g., to combinethe signals generated by the respective RM bias units), and/or forcoupling to control terminals 404 a and 404 b of RMU 400 (e.g., tocontrol an optical modulator 106).

Embodiments of RM bias unit 500 may use resistance modulation to controlan optical modulator 106 to optically encode data received on dataterminal 402. Embodiments of RM bias unit 500 may modulate a resistancebetween control line 410 and supply terminal 406, and/or a resistancebetween control line 411 and ground terminal 407. An RM bias unit mayselectively provide a voltage across its control lines 410 and 411, withthe higher potential on its control line 410. Likewise, an RM bias unit500 may selectively place its control lines in a high-impedance state,thereby effectively de-coupling the RM bias unit from RMU controlterminals 404 a and 404 b.

In the embodiment of FIG. 6, RMU 400 includes two resistance modulationbias units (RM bias units) 500 a and 500 b. RM bias units 500 a and 500b may selectively provide positive and negative voltages across controlterminals 404 a and 404 b of RMU 400. As can be seen, in the example ofFIG. 6, control lines 410 a and 411 a of RM bias unit 500 a are coupledto the RMU's control terminals 404 a and 404 b, respectively. Bycontrast, control lines 410 b and 411 b of RM bias unit 500 b arecoupled to the RMU's control terminals 404 b and 404 a, respectively.Thus, RMU 400 of FIG. 6 may provide a positive voltage across controlterminals 404 a and 404 b by coupling RM bias unit 500 a to the controlterminals and de-coupling RM bias unit 500 b from the control terminals(e.g., by placing control lines 410 b and 411 b in a high-impedancestate). Likewise, RMU 400 may provide a negative voltage across controlterminals 404 a and 404 b by coupling RM bias unit 500 b to the controlterminals and de-coupling RM bias unit 500 a from the control terminals(e.g., by placing control lines 410 a and 411 a in a high-impedancestate).

FIG. 7 shows a block diagram of resistance modulation bias unit (RM biasunit) 500, according to some embodiments. In the embodiment of FIG. 7,RM bias unit 500 includes a resistance modulation control unit (RMcontrol unit) 602 and a resistance modulation drive unit (RM drive unit)604, which includes one or more resistance modulation drive segments (RMdrive segments) 605.

In some embodiments, RM control unit 602 may have a data terminal 402, aconfiguration terminal 440, and a drive unit control terminal 606. Dataterminal 402 and its associated functions are described above withreference to FIG. 4. Configuration terminal 440 may be coupled toconfiguration terminal 408 of RM bias unit 500 and may be for receivingconfiguration signals associated with programmable control of the RMcontrol unit's operation. In some embodiments, the configuration signalsreceived on configuration terminal 440 may control the delays (e.g.,switching delays or propagation delays) of one or more circuit elementsin RM control unit 602. In some embodiments, increasing the delays ofone or more circuit components may decrease the per-bit energydissipation of the optical modulation sub-system. Likewise, decreasingthe delays of one or more circuit components may increase the opticalmodulation sub-system's per-bit energy dissipation.

RM control unit 602 may control the operation of RM drive unit 604 basedon data to be optically encoded, so that RM drive unit 604 either placesits control lines 410 and 411 in a high-impedance state, or provides asignal on control lines 410 and 411 suitable for controlling opticalmodulator 106 to optically encode the data. In some embodiments, RMcontrol unit 602 may receive data to be optically encoded via dataterminal 402. Based on the data to be optically encoded, the RM controlunit 602 may generate a drive unit control signal, which is provided toRM drive unit 604 via drive unit control terminal 606. In someembodiments, RM control unit 602 may include a pulse generator circuitthat converts the data to be optically encoded into pulses that controlsthe operation of RM drive unit 604.

As shown in FIG. 7, RM drive unit 604 may have an input terminal 607, asupply terminal 406, a ground terminal 407, a configuration terminal441, and control lines 410 and 411. The supply terminal 406, groundterminal 407, control lines 410 and 411, and their associated functionsare discussed above with respect to FIGS. 4 and 6. Configurationterminal 441 may be coupled to configuration terminal 408 of RM biasunit 500 and may be for receiving configuration signals associated withprogrammable control of the RM drive unit's operation.

In response to the drive unit control signal received on input terminal607, embodiments of RM drive unit 604 may perform resistance modulationto control an optical transmitter 106 to optically encode data receivedby the RM control unit 602. RM drive unit 604 may perform suchresistance modulation using one or more RM drive segments 605. Theresistance between the RM drive unit's control line 410 and supplyterminal 406 may depend on the resistances between the one or more RMdrive segments' control terminals 420 and supply terminals 406. Forexample, the resistance of the RM drive unit 604 between control line410 and supply terminal 406 may be the parallel combination of theresistances of the one or more RM drive segments 605 between theirrespective control terminals 420 and supply terminal 406. Likewise, theresistance of the RM drive unit 604 between control line 411 and groundterminal 407 may be the parallel combination of the resistances of theone or more RM drive segments 605 between their respective controlterminals 421 and ground terminal 406.

In response to the drive unit control signal provided by RM control unit602, an RM drive segment 605 may modulate a resistance between controlterminal 420 and supply terminal 406, and/or a resistance betweencontrol terminal 421 and ground terminal 407. In some embodiments, RMdrive segment 605 may include a level-shifter circuit.

Embodiments of modulator controller 400 may perform resistancemodulation to adjust the drive strength of optical modulator 106. Byenabling and disabling RM bias units and/or RM drive segments, modulatorcontroller 400 may adjust (modulate) the resistance between its controlterminals (404 a, 404 b) and a supply, or the resistance between itscontrol terminals and ground. For example, based on configurationsignals received via configuration terminal 408, RMU 400 may activate ordeactivate features such as pre-emphasis and priming, and/or control theshape of the optical modulator's drive strength characteristic. Suchadjustments may affect the modulation sub-system's bit rate or per-bitenergy dissipation. These programmable features may be implemented byactivating or deactivating one or more RM bias units 500. The controllines (410, 411) of a deactivated RM bias unit 500 may be placed in ahigh-impedance state, thereby effectively decoupling the deactivated RMbias unit 500 from the RMU's control terminals (404 a, 404 b).

As another example, the resistance modulation performed by RM drive unit604 may be configurable in response to configuration signals received byRMU 400 on terminal 408, which may be coupled to an RM drive unit'sconfiguration terminal 441. In some embodiments, based on suchconfiguration signals, RM drive unit 604 may activate or deactivate oneor more RM drive segments 605, thereby adjusting the resistance betweencontrol line 410 and supply terminal 406, or the resistance betweencontrol line 411 and ground terminal 407. In this manner, RM drive unit604 may adjust the drive strength of an optical modulator 106 to whichRMU 400 is coupled. The control terminals (420, 421) of a deactivated RMdrive segment 605 may be placed in a high-impedance state, therebyeffectively decoupling the deactivated RM drive segment 605 from the RMdrive unit's control lines (410, 411).

As described above, embodiments of RM control unit 602 may generatepulse signals in response to data received via data terminal 402. Thedurations and timing (e.g., delays) of the pulses may be configurable.FIGS. 8A-8D show schematics of RM control units 602, according to someembodiments. The circuits illustrated in FIGS. 8A-8D are non-limitingexamples of pulse-generator circuits, and embodiments are not limited tothe illustrated circuits. RM control unit 602 may be implemented, forexample, using any circuit known to one of ordinary skill in the art orotherwise suitable for generating a pulse.

FIG. 8A shows a schematic of an RM control unit 602 a, according to someembodiments. In the embodiment of FIG. 8A, RM control unit 602 a may beconfigured to generate a negative-going pulse (e.g., an active-lowsquare wave) in response to a rising edge in the data signal receivedvia data terminal 402. As can be seen, the RM control unit 602 a of FIG.8A may include an inverter chain (e.g., two or more series-coupledinverters 804) and a NAND gate 806. In some embodiments, the number ofinverters in the inverter chain may be odd. Embodiments of control unit602 a may be suitable for activating a drive unit 604 for a sub-bitperiod following a low-to-high transition (e.g., a transition from a0-bit to a 1-bit) in the data to be encoded by optical modulator 106.

The duration of the pulse generated by RM control unit 602 a may dependon the delay through each inverter 804 and on the number of inverters inthe inverter chain. FIG. 9 shows a schematic of an inverter 804,according to some embodiments. In some embodiments, the delay throughinverter 804 may be configurable. In the example of FIG. 9, inverter 804includes multiple p-channel MOSFETs 906 and multiple n-channel MOSFETs908. The parallel-connected p-channel MOSFETs and the parallel connectedn-channel MOSFETs may be controlled by configuration signals receivedvia configuration terminal 440 of the RM control unit. As the number ofswitched-on pFETs in the set of parallel-connected pFETs increases, thepull-up strength of inverter 804 may increase, thereby decreasing thedelay through the inverter on a transition from a low output to a highoutput. Likewise, as the number of switched-on nFETs in the set ofparallel-connected nFETs increases, the pull-down strength of inverter804 may increase, thereby decreasing the delay through the inverter on atransition from a high output to a low output. The inverter's powerdissipation may also increase as the number of switched-on FETsincreases.

In some embodiments, one or more of the inverters 804 of RM control unit602 a may be implemented using inverters with configurable delays, suchas the inverter of FIG. 9. In such embodiments, the duration of thepulse generated by the RM control unit 602 a may be adjusted byadjusting the delays through the inverters. In some embodiments,decreasing the duration of the pulse by reducing the delays through theinverters may result in a corresponding increase in the powerdissipation of RM control unit 602 a.

FIG. 8B shows a schematic of an RM control unit 602 b, according to someembodiments. In the embodiment of FIG. 8B, RM control unit 602 b may beconfigured to generate a negative-going pulse (e.g., an active-lowsquare wave) for the duration of a high data signal (e.g., a 1-bit)received via data terminal 402. As can be seen, the RM control unit 602b of FIG. 8C may include an inverter 804. Embodiments of control unit602 b may be suitable for activating a drive unit 604 for a bit period(or a multi-bit period) following receipt of a 1-bit (or multipleconsecutive 1-bits) in the data to be encoded by optical modulator 106.

FIG. 8C shows a schematic of an RM control unit 602 c, according to someembodiments. In the embodiment of FIG. 8C, RM control unit 602 c may beconfigured to generate a delayed positive-going pulse (e.g., anactive-high square wave) in response to a falling edge in the datasignal received via data terminal 402. The pulse may return to a lowlevel in response to a rising edge in the data signal. As can be seen,the RM control unit 602 c of FIG. 8C may include an inverter chain(e.g., two or more series-coupled inverters 804), a NOR gate 808, an ORgate 810, and an inverter 804. In some embodiments, the number ofinverters in the inverter chain may be odd. Embodiments of control unit602 c may be suitable for activating a drive unit 604 for a bit period(or a multi-bit period) following receipt of a 0-bit (or multipleconsecutive 0-bits) in the data to be encoded by optical modulator 106.In some embodiments, the duration of the delay between a high-to-lowtransition in the data signal and the activation of the pulse may beadjusted by adjusting the delays through the inverters in the inverterchain, in the manner described above.

FIG. 8D shows a schematic of an RM control unit 602 d, according to someembodiments. In the embodiment of FIG. 8D, RM control unit 602 d may beconfigured to generate a negative-going pulse (e.g., an active-lowsquare wave) in response to a falling edge in the data signal receivedvia data terminal 402. As can be seen, the RM control unit 602 d of FIG.8D may include an inverter chain (e.g., two or more series-coupledinverters 804) and a NOR gate 808. In some embodiments, the number ofinverters in the inverter chain may be odd. Embodiments of control unit602 d may be suitable for activating a drive unit 604 for a sub-bitperiod following a 1-to-0 transition in the data to be encoded byoptical modulator 106. In some embodiments, the duration of the pulsegenerated by the RM control unit 602 d may be adjusted by adjusting thedelays through the inverters in the inverter chain, in the mannerdescribed above.

As described above, embodiments of RM drive segments 605 may performresistance modulation in response to drive unit control signals (e.g.,pulses) received from RM control unit 602. FIGS. 10A-10B show schematicsof RM control segments 605, according to some embodiments. In someembodiments, RM control segment 605 may be implemented using a tri-statecircuit (i.e., a circuit configured to selectively provide high output,low output, or high-impedance output).

FIG. 10A shows a schematic of an RM drive segment 605 a, according tosome embodiments. In the embodiment of FIG. 10A, RM drive segment 605 amay be configured to pull control terminal 420 up to a voltage providedby supply terminal 406 (e.g., an I/O supply voltage HVDD, a core logicsupply voltage VDD, or a voltage derived from the I/O supply voltage orthe core logic supply voltage) when the drive unit control signalprovided at input terminal 607 is low. In addition, RM drive segment 605a may be configured to pull control terminal 421 down to ground when thedrive unit control signal provided at input terminal 607 is low.Furthermore, RM drive segment 605 a may be configured to place controlterminals 420 and 421 in a high-impedance state when the drive unitcontrol signal provided at input terminal 607 is high, and/or when theconfiguration signal provided at configuration terminal 451 is low.

FIG. 10B shows a schematic of an RM drive segment 605 b, according tosome embodiments. In the embodiment of FIG. 10B, RM drive segment 605 bmay be configured to pull control terminal 420 up to a voltage providedby supply terminal 406 (e.g., an I/O supply voltage HVDD, a core logicsupply voltage VDD, or a voltage derived from the I/O supply voltage orthe core logic supply voltage) when the drive unit control signalprovided at input terminal 607 is high. In addition, RM drive segment605 a may be configured to pull control terminal 421 down to ground whenthe drive unit control signal provided at input terminal 607 is high.Furthermore, RM drive segment 605 a may be configured to place controlterminals 420 and 421 in a high-impedance state when the drive unitcontrol signal provided at input terminal 607 is low, and/or when theconfiguration signal provided at configuration terminal 451 is low.

FIG. 11A shows a schematic of a modulator driver circuit 208, accordingto some embodiments. In some embodiments, the modulator driver circuit208 of FIG. 11A may be implemented as a digital push-pull circuit withconfigurable drive strengths, capable of performing sub-bit pre-emphasisand/or priming. The embodiment of FIG. 11A may be suitable forcontrolling an optical modulator 106, which may include a ring resonatorembedded in a PIN diode. The embodiment of FIG. 11A may be configured topre-emphasize the optical modulator's drive strength during low-to-highand high-to-low transitions. The embodiment of FIG. 11A may also beconfigured to prime optical modulator 106 for a fast transition from a0-bit encoding state to a 1-bit encoding state. Embodiments of theoptical modulation system illustrated in FIG. 11A may operate at a highbit rate (e.g., 2.5 Gb/s) and with low energy dissipation (e.g., 1.2pJ/b).

In the embodiment of FIG. 11A, modulator driver circuit 208 includesfour resistance modulation bias units (RM bias units) 500 a-500 d. Thedata terminal 402 of each RM bias unit is for receiving data to beoptically encoded. The control terminals 410 and 411 of each RM biasunit are coupled to the control terminals 404 a and 404 b of opticalmodulator 106 in the manner illustrated. In some embodiments, opticalmodulator 106 may include a ring resonator embedded in a PIN diode, withcontrol terminal 404 a coupled to the diode's anode and control terminal404 b coupled to the diode's cathode.

In the embodiment of FIG. 11A, RM bias unit 500 a includes an RM controlunit 602 a, such as the RM control unit 602 a illustrated in FIG. 8A,and an RM drive unit 604 a with one or more (e.g., four) RM drivesegments 605 a, such as the RM drive segments 605 a illustrated in FIG.10A. RM bias unit 500 b includes an RM control unit 602 b, such as theRM control unit 602 b illustrated in FIG. 8B, and an RM drive unit 604 bwith one or more (e.g., two) RM drive segments 605 a, such as the RMdrive segments 605 a illustrated in FIG. 10A.

In some embodiments, the operation of RM bias units 500 a and 500 b maybe illustrated in FIG. 3C. When the data to be encoded by the opticalmodulator switches from a 0-bit to a 1-bit, modulator driver circuit 208may enter a mode of operation 322 in which RM bias units 500 a and 500 bcontrol the drive strength of optical modulator 106 according to theresistance they provide between the RMU's control terminals (404 a, 404b) and supply 406 or ground 407. In FIG. 3C, portion 350 of the drivestrength characteristic 300 c represents the steady-state drive strengthof optical modulator 106 when RM bias units 500 a and 500 b are active.As the concentration of charge carriers in the PIN diode's intrinsicregion increases, optical modulator 106 may enter encoding state 312(e.g., a 1-bit encoding state). Shortly after the data to be encodedswitches to a 1-bit, modulator driver circuit 208 may enter a mode ofoperation 324 in which the control lines of RM bias unit 500 a are in ahigh-impedance state, and RM bias unit 500 b controls the drive strengthas indicated by portion 342 of signal 300 c. Modulator driver circuit208 may remain in mode of operation 324, and optical modulator 106 mayremain in encoding state 312, at least until the data to be encodedswitches to a 0-bit. When the data to be encoded switches to a 0-bit, RMbias unit 500 b may place its control terminals in a high-impedancestate. In other words, RM bias units 500 a and 500 b may collectivelypre-emphasize the optical modulator's drive strength when the data to beencoded switches from a 0-bit to a 1-bit, thereby forward-biasing thePIN diode, and RM bias unit 500 b may continue to forward-bias the PINdiode while the data to be encoded remains a 1-bit.

In the embodiment of FIG. 11A, RM bias unit 500 d includes an RM controlunit 602 d, such as the RM control unit 602 d illustrated in FIG. 8D,and an RM drive unit 604 d with one or more (e.g., one) RM drive segment605 b, such as the RM drive segments 605 b illustrated in FIG. 10B. RMbias unit 500 c includes an RM control unit 602 c, such as the RMcontrol unit 602 c illustrated in FIG. 8C, and an RM drive unit 604 cwith one or more (e.g., two) RM drive segments 605 a, such as the RMdrive segments 605 a illustrated in FIG. 10A.

In some embodiments, the operation of RM bias units 500 d and 500 c maybe illustrated in FIG. 3C. When the data to be encoded by the opticalmodulator switches from a 1-bit to a 0-bit, modulator driver circuit 208may enter a mode of operation 326 in which RM bias unit 500 d controlsthe drive strength of optical modulator 106 according to the resistanceit provides between the RMU's control terminals (404 a, 404 b) andsupply 406 or ground 407. In FIG. 3C, portion 351 of the drive strengthcharacteristic 300 c represents the steady-state drive strength ofoptical modulator 106 when RM bias unit 500 d is active. As theconcentration of charge carriers in the PIN diode's intrinsic regiondecreases, optical modulator 106 may enter encoding state 314 (e.g., a0-bit encoding state). Shortly after the data to be encoded switches toa 0-bit, modulator driver circuit 208 may enter a mode of operation 328in which the control terminals of RM bias unit 500 d are in ahigh-impedance state, and RM bias unit 500 c controls the drive strengthof optical modulator 106 as indicated by portion 352 of signal 300 c.Modulator driver circuit 208 may remain in mode of operation 328, andoptical modulator 106 may remain in encoding state 314, at least untilthe data to be encoded switches to a 1-bit. In other words, RM biasunits 500 d and 500 c may collectively pre-emphasize the opticalmodulator's drive strength when the data to be encoded switches from a1-bit to a 0-bit, thereby reverse-biasing the PIN diode, and RM biasunit 500 c may weakly forward-bias the PIN diode while the data to beencoded remains a 0-bit.

FIG. 11B shows a schematic of a modulator driver circuit 208, accordingto some embodiments. The embodiment of FIG. 11B may be suitable forcontrolling the drive strength of optical modulator 106, which mayinclude a ring resonator embedded in a PIN diode. The embodiment of FIG.11B may be configured to pre-emphasize the optical modulator's drivestrength during low-to-high transitions. Embodiments of the opticalmodulation system illustrated in FIG. 11B may operate at a high bit rate(e.g., 600 Mb/s).

In the embodiment of FIG. 11B, modulator driver circuit 208 includesthree resistance modulation bias units (RM bias units) 500 a, 500 b, and500 e. The data terminal 402 of each RM bias unit is for receiving datato be optically encoded. The control terminals 410 and 411 of each RMbias unit are coupled to the control terminals 404 a and 404 b ofoptical modulator 106 in the manner illustrated. In some embodiments,optical modulator 106 may include a ring resonator embedded in a PINdiode, with control terminal 404 a coupled to the diode's anode andcontrol terminal 404 b coupled to the diode's cathode.

The implementations and operations of RM bias units 500 a and 500 b aredescribed above with reference to FIG. 11A.

In the embodiment of FIG. 11B, RM bias unit 500 e includes an RM controlunit 602 b, such as the RM control unit 602 b illustrated in FIG. 8B,and an RM drive unit 604 e with one or more (e.g., one) RM drive segment605 b, such as the RM drive segments 605 b illustrated in FIG. 10B. Insome embodiments, the operation of RM bias unit 500 e may be illustratedin FIG. 3A. When the data to be encoded by the optical modulatorswitches from a 1-bit to a 0-bit, modulator driver circuit 208 may entera mode of operation 326 in which RM bias unit 500 d controls the drivestrength of optical modulator 106 according to the resistance itprovides between the RMU's control terminals (404 a, 404 b) and supply406 or ground 407. In FIG. 3A, portion 344 of the drive strengthcharacteristic 300 a represents the steady-state drive strength ofoptical modulator 106 when RM bias unit 500 e is active. As theconcentration of charge carriers in the PIN diode's intrinsic regiondecreases, optical modulator 106 may enter encoding state 314 (e.g., a0-bit encoding state). Modulator driver circuit 208 may remain in modeof operation 326, and optical modulator 106 may remain in encoding state314, at least until the data to be encoded switches to a 1-bit. In otherwords, RM bias unit 500 e reverse-bias the PIN diode while the data tobe encoded remains a 0-bit.

Embodiments of a Modulator Control Method

FIG. 12 shows a flowchart of a method of encoding a first symbol and asecond symbol in an optical signal, according to some embodiments. Theoptical signal may be carried by an optical waveguide, which may beformed from silicon, polysilicon, and/or another silicon-based material.An optical property of the waveguide, such as its refraction index, maydepend on a concentration of charge carriers in a portion of asemiconductor device, which may be a component of an optical modulator.For example, an optical property of the waveguide may depend on aconcentration of charge carriers in an intrinsic region (portion) of aPIN diode (semiconductor device) in which a ring resonator is embedded.

At act 1202 of the illustrated method, a first symbol may be encoded inthe optical signal. The first symbol may represent, for example, abinary digit, such as 1. However, embodiments are not limited in thisregard. In some embodiments, the first symbol may represent any digit ofan N-symbol code (where N is a finite number), such as a hexadecimaldigit of a 16-symbol code.

Encoding the first symbol in the optical signal may include injectingcharge carriers, at a first rate, into the portion of the semiconductordevice (e.g., the intrinsic region of the PIN diode). In someembodiments, injection of charge carriers at the first rate may beachieved by applying a forward bias voltage to the semiconductor device(e.g., PIN diode). The forward bias voltage may be any voltage thatexceeds a threshold voltage of the semiconductor device.

In some embodiments, encoding the first symbol in the optical signal mayalso include injecting charge carriers, at a third rate, into theportion of the semiconductor device (e.g., the intrinsic region of thePIN diode). In some embodiments, injection of charge carriers at thethird rate may be achieved by applying a forward bias voltage to thesemiconductor device (e.g., PIN diode). This forward-bias voltage may bea voltage that is greater than a threshold voltage of the semiconductordevice.

The injection of charge carriers at the third rate may occur prior tothe injection of charge carriers at the first rate. In some embodiments,the injection of charge carriers at the third rate may occur immediatelyprior to the injection of charge carriers at the first rate. The thirdrate of charge carrier injection may be greater than the first rate ofcharge carrier injection. In some embodiments, the drive strength of thesemiconductor device during injection of charge carriers at the thirdrate may exceed the drive strength of the semiconductor device duringinjection of charge carriers at the first rate. Thus, injection ofcharge carriers in this manner may correspond to pre-emphasis of thesemiconductor component's drive strength.

At act 1204 of the illustrated method, a second symbol may be encoded inthe optical signal. The second symbol may represent, for example, abinary digit, such as 0. However, embodiments are not limited in thisregard. In some embodiments, the second symbol may represent any digitof an N-symbol code (where N is a finite number), such as a hexadecimaldigit of a 16-symbol code.

Encoding the second symbol in the optical signal may include injectingcharge carriers, at a second rate, into the portion of the semiconductordevice (e.g., the intrinsic region of the PIN diode). In someembodiments, injection of charge carriers at the first rate may beachieved by applying a forward bias voltage to the semiconductor device(e.g., PIN diode). The forward bias voltage may be a weak forward-biasvoltage (e.g., a forward voltage that does not exceed a thresholdvoltage of the semiconductor device). In some embodiments, the drivestrength of the semiconductor device during injection of charge carriersat the first rate may exceed the drive strength of the semiconductordevice during injection of charge carriers at the second rate.

In some embodiments, encoding the second symbol in the optical signalmay also include preventing injection of charge carriers into, ordepleting charge carriers from, the portion of the semiconductor device(e.g. the intrinsic region of the PIN diode). In some embodiments,preventing injection of charge carriers or depleting charge carriers maybe achieved by applying a reverse bias voltage to the semiconductordevice (e.g., PIN diode). This reverse-bias voltage may be a voltagethat is less than a zero volts (e.g., a voltage with a higher potentialat the cathode of the semiconductor device and a lower potential at theanode of the semiconductor device).

The prevention of injection or the depletion of charge carriers mayoccur prior to the injection of charge carriers at the second rate(e.g., immediately prior). In some embodiments, the drive strength ofthe semiconductor device during prevention of injection or duringdepletion of charge carriers may be less than the drive strength of thesemiconductor device during injection of charge carriers at the secondrate. Thus, the prevention of injection or the depletion of chargecarriers in this manner may correspond to pre-emphasis of thesemiconductor component's drive strength.

While some above-described embodiments relate to modulating an opticalsignal by using a PIN diode to control injection of charge carriers intoa region of a semiconductor, other embodiments are not limited in thismanner. In some embodiments, an optical signal may be modulated by usinga carrier-concentration controller, such as a PN-junction diode or a MOScapacitor, to control injection, depletion, or accumulation of chargecarriers in a region of a semiconductor.

Embodiments of an Integrated Circuit Fabrication Method

FIG. 13 illustrates components of an optical data communication system(e.g., a silicon photonic system) fabricated in layers of a standardCMOS process, according to some embodiments. In some embodiments, thestandard CMOS process may be silicon-on-insulator (SOI) process whichincludes a buried-oxide layer 1305, a body-silicon layer 1306, agate-polysilicon layer 1307, a first metal layer 1308, one or moreoxide/nitride/low-k dielectric layers 1309, and one or more additionalmetal layers 1310, which are fabricated on a wafer 1304 (e.g., a siliconhandle wafer).

In the embodiment of FIG. 13, an nFET 1301, a pFET 1302, and an opticalwaveguide 1321 are fabricated in the body-silicon layer 1306 and thegate-polysilicon layer 1307 of the standard CMOS process. The nFET 1301may include a p-doped silicon region 1310, n+ diffusion regions 1311 and1312, and an n+ polysilicon gate 1313. The pFET 1302 may include ann-doped silicon region 1330, p+ diffusion regions 1331 and 1332, and ap+ polysilicon gate 1313. In some embodiments, strained silicontechniques may be used to fabricate the nFET 1301 and/or pFET 1302. Insome embodiments, the gate and diffusion regions of the pFET and/or nFETmay be capped with a layer of nickel silicide (NiSi) 1314.

In some embodiments, the optical waveguide may include a body-siliconwaveguide core portion 1322 and/or a strip-loaded waveguide core portion1323, both of which may be fabricated in the body-silicon process layer1306 with the bodies of the nFET 1301 and pFET 1302. In someembodiments, the optical waveguide may include a polysilicon waveguidecore portion 1324, which may be fabricated in the gate-polysilicon layerprocess layer 1307 with the gates of the nFET 1301 and pFET 1302.

In some embodiments, the thicknesses of the buried-oxide layer 1305,body-silicon layer 1306, and gate-polysilicon layer 1307 may be, forexample, 100-200 nm, 80-120 nm, and 65-100 nm. However, embodiments arenot limited in this regard. In some embodiments, the thicknesses of thevarious processing layers may be the process layer thicknessesassociated with standard CMOS processing nodes (e.g., the process layerthicknesses associated with a 130 nm node, a 90 nm node, a 65 nm node, a45 nm node, a 32 nm node, a 22 nm node, a 14 nm node, a 10 nm node, a 7nm node, or a 5 nm node).

Embodiments of a standard CMOS process are not limited to the exemplarySOI process described above and illustrated in FIG. 13. In someembodiments, the standard CMOS process may be, for example, a gate-firstSOI process or a gate-last SOI process. In some embodiments the standardCMOS process may be, for example, a bulk process (e.g., a gate-firstbulk process or a gate-last bulk process). In some embodiments, thenumber of supplies available in a standard CMOS process may be limited,for example, to 2-3. In some embodiments, the voltage headroom in astandard CMOS process may be restricted, for example to 1-2V, e.g., dueto transistor leakage and/or gate oxide and drain-source junction breakvoltages. In some embodiments, a process may not be a standard CMOSprocess if the process does not produce a chip comprising at least oneNMOS transistor and at least one PMOS transistor.

FIG. 14 shows a flowchart of a method of fabricating an integratedcircuit in a manufacturing process, according to some embodiments. Theintegrated circuit may be a monolithic integrated circuit, and themanufacturing process may be a standard CMOS process. For example, themanufacturing process may be a standard CMOS process in a sub-130 nmtechnology node, such as a 130 nm process, a 90 nm process, a 65 nmprocess, a 45 nm process, a 32 nm process, a 28 and/or 22 nm process, a14 nm process, a 11 nm process, a 7 nm process, or a 5 nm process.

At act 1402, in a first layer of the manufacturing process, a body of atransistor is fabricated. The layer of the manufacturing process may bea front-end body-silicon layer. The transistor may be a component of amodulator driver circuit configured to control an optical modulator. Theoptical modulator may use a semiconductor device, such as a PIN diodewith a ring resonator embedded in its intrinsic region, to encode datain optical signals carried by an optical waveguide. The transistor maybe configured to modulate a resistance between the optical modulator anda supply (e.g., a core logic power supply or an I/O power supply).

At act 1404, a gate of the transistor may be fabricated in a secondlayer of the manufacturing process. The layer of the manufacturingprocess may be a front-end gate-polysilicon layer.

At act 1406, an optical waveguide core may be fabricated in the firstand/or second layer of the manufacturing process. The waveguide core maybe a rib waveguide core or a strip waveguide core. In some embodiments,the transistor may modulate a drive strength of a semiconductor device(e.g., a PIN diode) that controls the encoding of optical data in anoptical signal carried by the waveguide core.

In some embodiments, the method may also include fabricating a diode inthe first layer of the manufacturing process. In some embodiments, thediode may be a PIN diode and a portion of the optical waveguide core maybe embedded in a portion of the PIN diode, such as the intrinsic region.In some embodiments, the diode may be a PN junction diode and a portionof the optical waveguide core may be embedded in a portion of thePN-junction diode.

Additional Embodiments

Embodiments of the devices and techniques described herein may be usedin a broad range of applications, including but not limited to on-chipoptical links, chip-to-chip optical links, multi-socket processorcoherency traffic interfaces, processor-to-DRAM interfaces, networkrouters, field-programmable gate arrays (FPGAs), circuit boardinterconnects, backplane interconnects, rack interconnects, data centerinterconnects, and digital system interconnects (e.g., Fibre-Channel,PCIExpress, GigabitEthernet, etc.).

Described herein are embodiments of an N-symbol optical modulator whichmodulates one or more properties of light to encode symbols from anN-symbol alphabet. In some embodiments, an N-symbol optical modulatormay “actively modulate” (e.g., modify) one or more properties of lightto encode a symbol (e.g., if the properties of the light provided to theoptic modulator do not already correspond to the desired symbol), or may“passively modulate” one or more properties of the light to encode asymbol (e.g., if the properties of the light provided to the opticalmodulator already correspond to the symbol, such that modification isunnecessary). The term “modulate,” as used herein, encompasses bothactive and passive modulation.

References are made above to ‘high’ and ‘low’ signals or signals with‘high’ and ‘low’ values. One of ordinary skill in the art willunderstand that digital electronic devices discriminate between signalvalues that correspond to binary digits 0 and 1, and that the term‘high’ typically refers to signals that correspond to binary digit 1,while the term ‘low’ typically refers to signals that correspond tobinary digit 0. Embodiments are not limited by the voltages or othersignal values by which high signals are distinguished from low signals.

Embodiments are not limited to monolithic integration in a standard CMOSprocess. In some embodiments, the devices described herein may befabricated in a non-standard CMOS process, a heterogeneous fabricationprocess, or any other integrated circuit fabrication process known toone of ordinary skill in the art.

The term ‘light,’ as used herein, is not limited to visible light. Insome embodiments, the optical signals used for optical datacommunication may have wavelengths in the visible, near-infrared,infrared, and/or ultraviolet portions of the electromagnetic spectrum.In some embodiments, the optical signals of an optical datacommunication technology may have wavelengths of at least 400 nm. Insome embodiments, optical signals may have wavelengths of at least 1100nm. Embodiments are not limited in this regard.

The waveforms illustrated in FIGS. 3A-3C are non-limiting examples ofwaveform conditioning that embodiments of modulator driver circuit 208may perform. Embodiments of modulator driver circuit 208 may condition awaveform (e.g., drive strength signal waveform) of any suitablecarrier-concentration controller (e.g., PIN diode, PN-junction diode, orMOS capacitor) in any suitable way. Embodiments are not limited in thisregard.

Various aspects of the apparatus and techniques described herein may beused alone, in combination, or in a variety of arrangements notspecifically discussed in the embodiments described in the foregoingdescription and is therefore not limited in its application to thedetails and arrangement of components set forth in the foregoingdescription or illustrated in the drawings. For example, aspectsdescribed in one embodiment may be combined in any manner with aspectsdescribed in other embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

Various embodiments may be configured as described below:

-   (A1) A method of encoding a first symbol and a second symbol in an    optical signal carried by a waveguide, an optical property of the    waveguide depending on a concentration of charge carriers in a    portion of a semiconductor device, the method comprising: encoding    the first symbol in the optical signal, wherein encoding the first    symbol includes injecting charge carriers, at a first rate, into the    portion of the semiconductor device; and encoding the second symbol    in the optical signal, wherein encoding the second symbol includes    injecting charge carriers, at a second rate, into the portion of the    semiconductor device, wherein the first rate is greater than the    second rate.-   (A2) The method of (A1), wherein injecting charge carriers at the    first rate comprises applying a forward bias voltage to the    semiconductor device while controlling a drive strength of the    semiconductor device, the forward bias voltage exceeding a threshold    voltage of the semiconductor device.-   (A3) The method of any of (A1) to (A2), wherein injecting charge    carriers at the second rate comprises applying a forward bias    voltage to the semiconductor device while controlling a drive    strength of the semiconductor device, the forward bias voltage not    exceeding a threshold voltage of the semiconductor device.-   (A4) The method of any of (A1) to (A3), wherein: encoding the first    symbol further comprises performing pre-emphasis of charge carrier    injection by injecting charge carriers, at a third rate, into the    portion of the semiconductor device, prior to injecting charge    carriers at the first rate, and the third rate is greater than the    first rate.-   (A5) The method of (A4), wherein injecting charge carriers at the    third rate comprises applying a forward bias voltage to the    semiconductor device while controlling a drive strength of the    semiconductor device, the forward bias voltage exceeding a threshold    voltage of the semiconductor device, wherein the drive strength of    the semiconductor device during injection of charge carriers at the    third rate exceeds the drive strength of the semiconductor device    during injection of charge carriers at the first rate.-   (A6) The method of any of (A1) to (A5), wherein encoding the second    symbol further comprises depleting charge carriers from the portion    of the semiconductor device prior to injecting the charge carriers    at the second rate.-   (A7) The method of (A6), wherein depleting charge carriers from the    portion of the semiconductor device comprises applying a reverse    bias voltage to the semiconductor device.-   (A8) The method of any of (A1) to (A7), wherein: the semiconductor    device is a PIN diode, and the portion of the semiconductor device    is an intrinsic region of the PIN diode.-   (B1) An integrated circuit for controlling an optical modulator, the    optical modulator configured to encode data in an optical signal    carried by an optical waveguide, the optical modulator including a    semiconductor device, an optical property of the waveguide depending    on a concentration of charge carriers in a portion of a    semiconductor device, the integrated circuit comprising: a modulator    driver circuit configured to control encoding of a first symbol in    the optical signal by injecting charge carriers into a portion of    the semiconductor device at a first rate, and to control encoding of    a second symbol in the optical signal by injecting charge carriers    into the portion of the semiconductor device at a second rate,    wherein the first rate is greater than the second rate.-   (B2) The integrated circuit of (B1) wherein the modulator driver    circuit is configured to inject charge carriers at the first rate by    applying a forward bias voltage to the semiconductor device while    providing a first signal to the semiconductor device, the forward    bias voltage exceeding a threshold voltage of the semiconductor    device.-   (B3) The integrated circuit of any of (B1) to (B2), wherein the    modulator driver circuit is configured to inject charge carriers at    the second rate by applying a forward bias voltage to the    semiconductor device, the forward bias voltage not exceeding a    threshold voltage of the semiconductor device.-   (B4) The integrated circuit of any of (B1) to (B3), wherein the    modulator driver circuit is further configured to encode the first    symbol by pre-emphasizing injection of charge carrier injection,    pre-emphasizing injection of charge carriers comprises injecting    charge carriers, at a third rate, into the portion of the    semiconductor device, prior to injecting charge carriers at the    first rate, and the third rate is greater than the first rate.-   (B5) The integrated circuit of (B4), wherein the modulator driver    circuit is configured to inject charge carriers at the third rate by    applying a forward bias voltage to the semiconductor device while    providing a second signal to the semiconductor device, the forward    bias voltage exceeding a threshold voltage of the semiconductor    device, wherein a drive strength of the second signal exceeds a    drive strength of the first signal.-   (B6) The integrated circuit of any of (B1) to (B5), wherein the    modulator driver circuit is further configured to encode the second    symbol by depleting charge carriers from the portion of the    semiconductor device prior to injecting the charge carriers at the    second rate.-   (B7) The integrated circuit of (B6), wherein the driver modulator    circuit is configured to deplete charge carriers from the portion of    the semiconductor device by applying a reverse bias voltage to the    semiconductor device.-   (B8) The integrated circuit of any of (B1) to (B7), wherein: the    semiconductor device is a PIN diode, and the portion of the    semiconductor device is an intrinsic region of the PIN diode.-   (B9) The integrated circuit of any of (B1) to (B8), further    comprising the optical modulator and the optical waveguide.-   (B10) The integrated circuit of any of (B1) to (B9), wherein the    integrated circuit is a monolithic integrated circuit fabricated in    a standard CMOS process.-   (C1) A method of fabricating an integrated circuit in a    manufacturing process, comprising: in a first layer of the    manufacturing process, fabricating a body of a transistor of a    modulator driver circuit configured to control an optical modulator,    the transistor being configured to modulate a resistance between an    optical modulator and a supply; in a second layer of the    manufacturing process, fabricating a gate of the transistors of the    modulator driver circuit; and fabricating an optical waveguide core    in the first layer and/or the second layer of the manufacturing    process.-   (C2) The method of (C1), further comprising fabricating a diode in    the first layer of the manufacturing process, wherein a portion of    the optical waveguide core is embedded in a portion of the diode.-   (C3) The method of any of (C1) to (C2), wherein the manufacturing    process is a standard CMOS process.-   (C4) The method of any of (C1) to (C3), wherein the manufacturing    process is a 65 nm process, a 45 nm process, a 32 nm process, a 28    nm process, a 22 nm process, a 14 nm process, an 11 nm process, a 7    nm process, or a 5 nm process.-   (C5) The method of any of (C1) to (C3), wherein the manufacturing    process is a sub-90 nm process.-   (D1) An integrated circuit for controlling an optical modulator, the    optical modulator configured to encode data in an optical signal    carried by an optical waveguide, the optical modulator including a    semiconductor device, an optical property of the waveguide depending    on a concentration of charge carriers in a portion of a    semiconductor device, the integrated circuit comprising: a modulator    driver circuit including: a data terminal for receiving data to be    optically encoded; a supply terminal for coupling to one or more    supplies; a drive terminal for coupling to the optical modulator; a    resistive circuit coupled between the supply terminal and the drive    terminal, the resistive circuit including a plurality of portions;    and a control circuit coupled between the data terminal and the    resistive circuit, the control circuit being configured to modulate    a resistance of the resistive circuit by selectively coupling one or    more of the plurality of portions of the resistive circuit to the    drive terminal based on the data to be optically encoded.-   (D2) The integrated circuit of (D1), wherein: a first portion of the    control circuit is coupled to a first of the plurality of portions    of the resistive circuit, and a second of the plurality of portions    of the control circuit is coupled to a second of the plurality of    portions of the resistive circuit; and the first portion of the    resistive circuit is configured to provide a voltage of a first    polarity across the drive terminal, and the second portion of the    resistive circuit is configured to provide a voltage of a second    polarity across the drive terminal, the first and second polarities    being opposite polarities.-   (D3) The integrated circuit of (D2), wherein: the first portion of    the resistive circuit includes: two of more sub-circuits coupled in    parallel, and a configuration terminal for receiving a configuration    signal; and the modulator driver circuit is configured to modulate a    resistance of the first portion of the resistive circuit by    selectively coupling one or more of the two or more sub-circuits    between the drive terminal and a first of the one or more power    supplies based on the configuration signal.-   (D4) The integrated circuit of any of (D1) to (D3), wherein the    control circuit include a differential cascode voltage switch (DCVS)    circuit.-   (D5) The integrated circuit of any of (D1) to (D4), wherein the    control circuit includes one or more pulse generator circuits    configured to generate pulse signals in response to the data to be    optically encoded.-   (D6) The integrated circuit of any of (D1) to (D5), wherein the one    or more supplies include an input/output power supply and a core    logic power supply.-   (D7) The integrated circuit of any of (D1) to (D6), further    comprising the optical modulator and the optical waveguide.-   (D8) The integrated circuit of (D7), wherein: the semiconductor    device is a PIN diode, and the portion of the semiconductor device    is an intrinsic region of the PIN diode.-   (D9) The integrated circuit of any of (D1) to (D8), wherein the    integrated circuit is a monolithic integrated circuit fabricated in    a standard CMOS process.

What is claimed is:
 1. A method of fabricating an integrated circuit ina manufacturing process, comprising: in a silicon layer of themanufacturing process, fabricating a body of a transistor of a modulatordriver circuit configured to control an optical modulator, thetransistor being configured to modulate a resistance between an opticalmodulator and a supply, wherein the silicon layer is a first layer ofthe manufacturing process; in a second layer of the manufacturingprocess, fabricating a gate of the transistor of the modulator drivercircuit; fabricating an optical waveguide core in the silicon layer ofthe manufacturing process; and fabricating a diode in the silicon layerof the manufacturing process, wherein a portion of the optical waveguidecore is embedded in a portion of the diode.
 2. The method of claim 1,wherein the manufacturing process is a bulk silicon process.
 3. Themethod of claim 1, wherein the manufacturing process is a 65 nm process,a 45 nm process, a 32 nm process, or a 28 nm process.
 4. The method ofclaim 1, wherein the manufacturing process is a silicon-on-insulator(SOI) process.
 5. The method of claim 1, wherein the silicon layer isdoped.
 6. The method of claim 1, wherein the second layer of themanufacturing process comprises a poly-silicon layer.
 7. The method ofclaim 1, further comprising capping the second layer of themanufacturing process with a layer of nickel silicide.
 8. The method ofclaim 1, wherein the second layer of the manufacturing process has athickness that is between 65 nm and 200 nm.
 9. The method of claim 1,further comprising fabricating an optical resonator in the silicon layerof the manufacturing process.
 10. The method of claim 1, whereinfabricating the optical waveguide core comprises fabricating at leastone waveguide core selected from the group consisting of a rib waveguidecore and a strip waveguide core.
 11. The method of claim 1, whereinfabricating the diode comprises fabricating at least one diode selectedfrom the group consisting of a PIN diode and a PN junction diode.